The influence of porous adsorbent microstructure on selective adsorption from binary mixtures is explored via Monte Carlo simulations of a molecular model representing simple mixtures adsorbed in silica gel. Two adsorbent structures are considered : a matrix of adsorbent spheres arranged in an equilibrium hard sphere configuration, and a matrix of adsorbent spheres arranged in a face-centered cubic configuration. At low pressures toward the Henry’s law limit, microstructure strongly influences both the total adsorbate density and composition. At higher pressures where substantial filling of the adsorbent occurs, microstructure influences the total density much more than the composition. The principal source of departures from ideal solution behavior for the systems studied is interpreted to be coupled effects of adsorbate size differences and of heterogeneity in the adsorbate-adsorbent potential field.